Viruses have been shown to have utility in a variety of applications in biotechnology and medicine on many occasions. Each is due to the unique ability of viruses to enter cells at high efficiency. This is followed in such applications by either virus gene expression and replication and/or expression of an inserted heterologous gene. Thus viruses can either deliver and express genes in cells (either viral or other genes) which may be useful in for example gene therapy or the development of vaccines, or they may be useful in selectively killing cells by lytic replication or the action of a delivered gene in for example cancer.
Herpes simplex virus (HSV) has been suggested to be of use for the oncolytic treatment of cancer. Here the virus must however be disabled such that it is no longer pathogenic, i.e. does not replicate in and kill non-tumor cells, but such that it can still enter and kill tumor cells. For the oncolytic treatment of cancer, which may also include the delivery of gene(s) enhancing the therapeutic effect, a number of mutations to HSV have been identified which still allow the virus to replicate in culture or in actively dividing cells in vivo (e.g. in tumors), but which prevent significant replication in normal tissue. Such mutations include disruption of the genes encoding ICP34.5, ICP6, and thymidine kinase. Of these, viruses with mutations to ICP34.5, or ICP34.5 together with mutation of e.g. ICP6 have so far shown the most favourable safety profile. Viruses deleted for only ICP34.5 have been shown to replicate in many tumor cell types in vitro and to selectively replicate in artificially induced brain tumors in mice while sparing surrounding tissue. Early stage clinical trials have also shown their safety in man.
However, while promise has been shown for various viruses including HSV for the oncolytic treatment of cancer, the majority of this work has used virus strains which do not carry a heterologous gene which may enhance the anti-tumor effect. We propose that the combined use of HSV with an inactivating mutation in the gene encoding ICP34.5 together with the delivery of the gene encoding an immunomodulatory protein such as granulocyte macrophage colony stimulating factor (GM-CSF) encoded in the disabled virus genome may have optimal immune stimulating properties against the tumor to be treated, particularly if functions in the virus which usually reduce immune responses to HSV infected cells have also been inactivated. For example the HSV ICP47 protein specifically inhibits antigen presentation in HSV infected cells (Hill et al 1995), and the product of the UL43 gene and the vhs protein reduce the immune-stimulating abilities of dendritic cells infected with HSV. ICP47 and/or dendritic cell-inactivating genes might therefore usefully be deleted from an oncolytic HSV mutant virus used for the treatment of cancer, particularly if immune effects are to be enhanced through the use of GM-CSF or other immunostimulatory cytokine or chemokine GM-CSF has recently been shown to give an enhanced anti-tumor immune effect if expressed from within a tumor cell rather than administered systemically (Shi et al 1999). Thus in such use an oncolytic HSV mutant would be inoculated into a primary or secondary tumor where replication and oncolytic destruction of the tumor would occur. Immune responses would also be stimulated against the HSV infected cells, and also to tumor cells elsewhere which had spread from the primary tumor site.